Generally, multistage front chain gear bicycle assemblies comprise round chain gears different in tooth number and mounted to a crank means at the bicycle. The chain is shifted through a front derailleur from one chain gear to another for changing the bicycle speed.
Now, the front chain gear, while a cyclist on a saddle is pedalling the bicycle, is subjected to a constant load from a rear wheel through the chain and a rear chain gear, but a pedalling torque changes due to the position of the pedal, in turn a crank arm, so that the maximum torque is obtained in a region where each crank arm turns at an angle of about 70.degree..+-.5.degree. ahead of the upper dead point thereof and the minimum torque in the vicinity of the upper or lower dead point as shown in FIG. 3.
In a case of using a conventional round chain gear, in spite of the fact that the torque changes in one cycle period of pedalling, the angular velocity of the crank arm for rotating the front chain gear is constant, so that the cyclist's physical burden is heavy and the power in one cycle period of pedalling is lower in comparison with such heavy physical burden.
The inventor has proposed in Japanese patent application Ser. No. Sho 57-94,396 (U.S. patent application Ser. No. 497,051) a chain gear non-round but nearly elliptic, which changes the angular volocity of the pedal in one pedalling cycle period in consideration of a variation in torque and a speed pattern on the basis of inherent motion of the cyclist's legs like a swing of a pendulum. Specifically, the inventor has observed that if the peripheral speed in pedalling increases for the maximum torque, the power can be improved without increasing the physical burden on the cyclist, and if the same decreases for the torque, the physical burden can be reduced. Hence, the proposed chain gear is nonround but nearly elliptic and of different gear diameters and assembled with the crank means so that the gear diameter becomes a minimum of thereabout in the region where the maximum torque by pedalling is obtainable between both the upper and lower dead points of the crank arm and the same becomes a maximum or thereabout in the vicinity of the upper or lower dead point of crank arm where the minimum torque only is obtainable.
In other words, when each crank arm is positioned in the region where the maximum torque is obtained by pedalling to move the cyclist's legs faster due to the inherent motion thereof, the smaller gear-diameter portion of the chain gear corresponds to the crank arms to thereby increase the circumferential speed of the pedal. Conversely, when the crank arms are positioned in the vicinity of the upper and lower dead points so that the minimum torque only is obtained and the movement of the legs becomes slow, the larger diameter portion of the same corresponds to the crank arms so as to reduce the circumferential speed.
Accordingly, the power in one cycle period of pedalling is improved as a whole and the energy consumption of the cyclist reduces to perform smooth pedalling.
However, in a case where the non-round chain gear is used as a larger diameter chain gear at the multistage front chain gears, the chain, when shifted from a smaller diameter gear to the larger diameter one, may be excessively over-shifted by a chain guide, thereby creating a problem in that the chain often falls down outwardly beyond the larger diameter gear.
The reason for the above is that an interval 1 between the lower surface of the chain guide and the crest of each tooth at the larger pitch diameter portion is made proper (e.g., 1 to 5 mm) in consideration of preventing contact with both the members. Consequently, an interval between the same and the crest of each tooth at the smaller pitch diameter portion becomes inevitably larger than the above proper interval 1 by one-half of the difference between both the pitch diameters. Hence, an amount of over-shift of the chain guide necessary to shift the chain from the smaller diameter gear to the larger diameter one becomes excessive.